The quality of cork enclosures is determined by the presence and size of defects, voids, or cracks. These defects can harbor impurities, which in turn can cause cork taint. Cork taint typically spoils 3%-5% of all wines that use natural cork as a stopper.
Most corks are punched with the long axis of the cork perpendicular to the lenticels. The lenticels allow for the interchange of gases between the atmosphere and the interior structure of the cork tree. It has been shown that impurities do not quickly permeate down the length of the cork, but propagate much more quickly perpendicular to the long axis. Only when contaminated parts of the cork are in direct contact (approximately the first few millimeters of cork) with the contained liquid, such as wine, does transfer of contaminants, and cork taint, take place. The extraction and migration of non-volatile chemicals from cork depend on the local diffusion rates. Therefore, an understanding of the diffusion of liquid in cork is important. The presence of cracks, voids, and defects may increase diffusion. The presence of these potentially highly anisotropic features in the cork affects the measurement of absorbable contaminants such as trichloroanisole (TCA) in wine corks.
TCA is one of the primary contributors to off-flavors or cork taint in wine. The presence of TCA in natural cork is presently determined by a cork soak method in which cork samples are immersed in a neutral white wine which extracts or “releases” the TCA which has enough mobility in the cork structure to contaminate the wine. After a soak of typically 24 hours, the wine is then tested for the presence of TCA using gas chromatography. Herve, E., et al., ASEV Annual Meeting Reno, Nev.: http://www.corkqc.com/asev/asev2-2.htm.1999.
There are methods for determining the diffusion coefficient of water in natural corks. In one method, small cork samples are submerged in water. As a function of time, the dimensions and mass of the sample are measured to determine the change in volume and mass of the sample due to the diffusion of water into the cork. A second method involves inserting two metallic electrodes into the cork and measuring the electrical resistance between them. One side of the sample is placed in contact with a water reservoir. Over time, water diffuses from the reservoir into the cork's structure. When water is absent from the proximity of the electrodes, the resistance is very high. As the concentration of water increases between the two electrodes, the resistance dramatically decreases. These techniques have at least two inherent limitations: (a) they are inherently invasive detection methods and (b) they measure essentially average diffusion properties. In the cork submersion method, samples must be continually removed from the soak in order to record data. Using the electrical method, the cork structure must be punctured. Both methods measure average sample properties. For example, the cork submersion method measures the total volume and weight change of the sample. Some degree of localized measurement is possible with the electrical method by using multiple sets of electrodes. However, a spatially continuous measurement is not possible. The electrode method is particularly troublesome since the shape of the resistance versus time curves depends upon the depth of the electrode placement in the cork—an artifact of the inhomogeneity of the cork. Consequently, the electrode technique does not show an appreciable anisotropy in the radial and non-radial diffusion coefficient.
There is no visual indication that a particular cork prior to bottling will result in cork taint or that a previously bottled wine is tainted. Methods and apparatus for non-destructively evaluating diffusion characteristics of solid materials such as cork would be useful. Wine producers, bottlers, dealers, collectors and connoisseurs would greatly benefit from a non-destructive method and apparatus to determine the likelihood of a cork causing taint prior to the cork being used to seal a bottle and/or the likelihood a given cork in a sealed bottle would be a cause of taint.